The present invention relates to solutions of aluminum citrate.
Aluminum citrate solutions have been found useful for crosslinking polymers such as carboxymethylcellulose (CMC) or partially hydrolyzed polyacrylamide. The phenomenon has found application in enhanced oil recovery processes. An example of such is disclosed in U.S. Pat. No. 3,762,476. The in situ gelling of the polymers results in blocking of highly permeable zones which can allow one to recover oil which otherwise could not be recovered by conventional means.
In the past, dilute solutions have been prepared in the field by dissolving aluminum sulfate and sodium citrate in water. If the concentration of such solutions is increased, it has been noted that in some formations precipitates form as a result of interaction between the sulfate ions and alkaline earth metal cations in the formation or in the water employed. Such precipitations have been found undesirable since they can result in formation blockages where they are not wanted.
In view of the large quantities of aluminum citrate that are employed in such enhanced oil recovery processes, it is obviously desirable to be able to have available stable, highly concentrated solutions that are substantially free of particulate matter. It is generally desired to have an aluminum citrate solution which, after 100 hours at 100.degree. F., will not contain over about 3 weight percent of solid precipitates. Some such solutions have in the past been made by reacting aluminum chloride, citric acid, and either ammonia or sodium hydroxide.
More recently an alternative method has been discovered for making stable aqueous aluminum citrate solutions from aqueous sodium aluminate and either citric acid or sodium citrate. The technique is disclosed in U.S. patent application Ser. No. 525,911 filed Aug. 24, 1983, the disclosure of which is incorporated herein by reference. When an attempt was made to use this process to make larger batches of the aluminum citrate solutions by using a single reaction of larger quantities of the reactants it was observed that the formation of precipitates was a problem. Since it was known that low reaction temperatures were needed to minimize solid formation, it was concluded that the precipitation occurring during the preparation of larger batches was due to localized heat of reaction. Accordingly, it was concluded that agitation needed to be provided in order to minimize the possibility of localized heats of reaction that could not be dissipated sufficiently by cooling of the reaction mixture. Surprisingly, however, it was discovered that at higher levels of agitation the solids formation problem appeared again.
An object of the present invention is to provide a method for forming large batches of aluminum citrate solutions with a minimal amount of solids formation.